Color printing



1945- Q w. T. HANSON, JR. ET AL ,3

COLOR PRI'N'IING Filed June 10, 1942 EIREIFRIIYGENT VAR/ABLE JATURA /0r-/ CONSTANT HUE F/LTER.

POLAR/Z/NG AXIS NOT PARALLEL 7'0 E/THER B/REFR/NGENT AXIS.

Wes/g/ 771022500 Jr:

Ra4oh MEI/ans INVENTORS Wm W A TT '1 13/167.

" imit so t N orFlcE Wesley. T. Ransom Jr' and Ralph M. Evan'ai Rochester,- -N.'-Y., assignors to Eastman Kodak Company; Rochester, N; a corporation of New Jersey Application ti e 1c,- 1942, seria No. 446,444

7 This invention relates to color-printing and particularly tournethods of correcting color balance when printingfrom a multi-cclored original.

The correction "of color balance lsnot to be con- .fused with the type of. correction obtained by masking and in fact one embodiment of the present invention relates to a' combination of both types of color correction. Improper color-balance may be either in the original or in the distribu-- tion of sensitivity-in the s nsitive material on which the reproduction is being made. Therefore, one-properly speaks of the correct color balance of an original with respectto the color sensitive. material being printed; In S. Patent 2,256,385,

Evans and Luboshezdescribe one method of correcting color balance and the present invention may be considered as a, .direct improvement of that published methodi ltwill be noted that Evans and Luboshez required a-large number of color filters, but even then didnot have continuous'increments of color. v v

Itis anobject of the present invention topropassed through both,filters.

vide a method of controlling'color balance when reproducing a multicolored original. It is the main object of the invention to provide such a control giving continuous increments of control.

It is also an object of the invention to eliminate the need for a large number of different filters to be used alternatively or in combination to get the various stages of control as employedin the prior art. At this'point, it is noted that a wedged color filter is not satisfactory in any practical printing system because it interferes with uniformity of light'distribution; a

- These main objects are accomplished by employing aconstant hue variable saturation color filter and a preferred embodiment of the invention provides a method in which the hue can be varied from one hue, through neutral to the complementary hue.

the two Ifiltersin either order and-these filters may be anywhere between the light source and the photosensitivesurfaca .That is; either or both )may be in the beam illuminating the origigllba tween the original and the lens if :3. pr ection system is used or between the lens-and the sensitive, surface. In one particular embodiment described in detail below, the 'color filter must face the polarizing filter, and hence, it is turned around if the order o the two filters is reversed. Also, in the embodiment combining the present invention with that of our copending application, the order of the filters is of some importance, as will be noted. Since the color .filter may come either before or after the polarizing filter in the optical system, thee'fiective color saturation thereof. is not apparent until after theflight has Therefore, in the above description of the invention, we refer merely to the fact that the filter has variable satura-' tion in accordance with the light by which it isviewed rather than in accordance with the light incidentthereon. I I

At least two entirely different type of such variable saturation color'filters are well'known to those skilled in the study of polarized light. It

will be noted that either form iS quite diflerent from the combination of an ordinary color filter and two polarizing filters since this latter combination gives constant hue and constant satuuration with variable density.' One form of variable saturation filter consists of a vecto-dye filter in which the molecules or crystals of the dye-are It is a further object of a preferred embodiment of the invention to combine thismethod of color balance control withthe control of masking light give intermediate degreesof, saturation, but

the hue remains constant. With this arrangement there is a slight'increasein density corresponding to the increase in saturation as the plane or polarizationis rotated.

-A second type of variable saturation filter con- I sists of a, birefringent layer in combination with a second polarizing layer whosevibration. axis is oriented somewhere between the two vibration axes of the birefringent layer. Furthermore, the birefringent layer must be between the two p0- larizing filters. Obviously, the birefringent layer .'may be considered as combined with either polarizing layer, but for convenience it will be considered always with respect to one of the layers,

oto-

graphmasktogive the combination constituting a variable saturation constant hue filter. The saturation of this latter type of filter varies, as the first filter is rotated, from a maximum saturation for one hue through neutral to a maximum saturation for the complementary hue. Of course, complementary hues may be considered as algebraically the same, one being negative with respect to the other, but it is not usual to do so. However, at least over a range of orientations of the polarization plane, such a filter has constant hue varying from zero to a maximum. With this type of filter the-change from neutral through a maximum of one hueand back to neutral requires a rotation of'the first polarizing filter through 90, a second 90 carrying the color through'a maximum saturation of the complementary hue, On the other. hand, with the vectodye type or filter, a rotation of 180 is required to change from neutral to a maximum saturation and back to neutral.

. When using the birefringent plus polarizer type of saturation filter it is preferable to have the axes of the birefringent material at 45 to the axis of the fixed or second polarizer. Of course, these axes must beat some angle to the fixed polarizer;

otherwise the combination will always remain neutral and vary only in density as the first polarizer is rotated; With the vibration axes at 45.

, to the fixed polarizer axis, the change in density varied right through zero and to a maximum of the complementary hue which, of course, has its advantages, in color printing processes in which the color balance may be off either way. The density of a vectodye filter is usually less than the combination of a birefringent layer and fixed polarizer so that this embodiment has certain advantages. On the other hand, the birefringent .type can-give complementary hues and that form of birefringent type having the vibration axes at 45- to the fixed polarizer axes the density as well as the hue remains Dracti cally constant as the first polarizer is rotated. I

In practice the polarizer and the variable saturation filter arerotated relative to one another to give the saturation which corrects the color picture relative tothe photosensitive surface 'on which a print is being made. Of course, addiproduction is being made.

In our above mentioned copending application, a vectograph mask iscornbined with a multicolored original so that the degree of masking may be varied by adjusting the orientation of the printing light. 'Thatinvention may be readily combined with the present invention by first ad- .lusting the polarizing filter relative to the vecto quired and then adj the'variable satura e degree of masking retion color filter relative to the polarizing filter to give the saturation required to correct the color balance of the combination of the original and its mask relative to the sensitivity of the photosensitive surface upon which a print is being made. In this combination of the two inventions, the polarizer should be between the mask and the variable saturation color filter. That is, if the polarizing filter ls-between the source and the masked original, the color filter should be between the source and the polarizing filter; if

the polarizing filter is between the masked origi- I nal and the sensitive layer, the color filter should be between the polarizing filter and the sensitive layer. The invention and its advantages will be fully understood from the following description when read in connection with the accompanying drawing in which: Figs. 1 and 1A illustrate one form of the prior art. Fig. 2 illustrates apreferred embodiment of the invention.

Figs. 3 and 4 show two forms-of variable saturation color filters.

Fig. 5 illustrates a combination of the present invention and that of our copending application, Serial No. 446,445, filed concurrently herewith. r

In Fig. 1 light from a source it passes through amulticolored transparency Ii to print there- 'from by projection by a lens 12 onto a photosensitive surface l3 which may be developed to anatural color reproduction of the transparency II. The transparency ll may be either a negativeor a positive and similarly the photosensitive layer l3 may be processed either directly or by reversal. If the color balance of the transparency II or the balance of the sensitivity of the material I 3 is incorrect, one withrespect to the other, this may be corrected by inserting a color filter it having the correct hue and saturation. In practice it is desirable to make this correction very accurately, and hence, as illusshown in Fig. 2, the printing light. passes throu h a polarizing filter 2| and a-constanthue variable color saturation color filter 20, which filters are rotatable relative to one another. The fil-. tors are shown immediately adjacent to the lens I12, but either may be anywhere in the optical 0 system between the source Ill and the photosensitive surface l3. If no color bezlanc correction is required, the filters are rotated relative to one another to give a neutral hue. Adjust merits from this setting may-be made to any desired saturation, up tothe maximum available, by rotating either filter.

One form of variable saturation filter is illusj trated in Fig. 3 and consists of a second polarizing filter 23'and abirefringent layer 22 between 7 this second polarizerk23 and first polarizer 2!.

In the' arrangement shown, the color filter follows thepolarizingfilter 2|, but if it were'placed optically ahead the polarizer 2!, it should be (6 follows the polarizer '23 and therefore comes bet-urned around Iso that the birefringent layer 22 

